Lignocellulosic materials such as wood are renewable and sustainable alternative resources for the production of fuels and plastics [47]. Accelerated industrial development around the globe has resulted in strong demand for petroleum as a fuel and also as a source for plastics and chemicals. Lignocellulosic feedstocks present an alternative to alleviate some of this pressure on petroleum resources in a particularly sustainable way. They present a significantly carbon neutral solution since biomass sequesters atmospheric carbon during its growth phase which is released during its combustion. Moreover, lignocellulosics present a source of fuels such as ethanol relieving the stress on corn, grain and such agricultural food sources. Societal awareness of such positive environmental benefits in addition to its obvious economic advantages has made the development and implementation of biobased energy from lignocellulosics imperative. Biomass processing is expected to occur in large biorefineries manufacturing a spectrum of fuel, chemical and material products in a scale efficient manner.
One class of biomass processes begins by hydrolyzing wood or the lignocellulosic raw material under different temperature and pressure conditions using dilute acid, hot water or mild alkaline solutions. The lignocellulosic hydrolyzates produced by this process consist of dissolved and colloidal oligomers of hemicelluloses, lignin and small quantities of extractives. The hemicelluloses in the hydrolyzates are transformed into biofuels or biobased plastic products by fermentation or other routes. Hydrolyzates must however be significantly purified and detoxified in order to conduct and maximize yields of the downstream fermentation processes.
Lignocellulosics are some of the most sustainable and renewable feedstocks for energy and materials in the future [1]. Woody biomass is a particularly attractive source because of its higher density and potential for integration with existing pulp and paper mill operations. Since hardwoods are rich in xylans and acetyl groups, pretreatment processes using aqueous solutions produces hydrolyzates which can be fermented to produce bioethanol and biobutanol. Pretreatment involves a variety of hydrolysis processes using mineral acids, mild alkalis or autohydrolysis using hot water and many of these have been investigated in integrated biorefinery processes [2]. The hydrolyzate solutions produced by such pretreatment processes are considerably complex, containing particulates, colloidal substances, dissolved and colloidal polymers from the carbohydrate and lignin solubilization reactions. In addition, low molecular weight organics such as acetic acid, methanol, furans and aromatics occur in the solution. A number of the compounds in the hydrolyzates are potent fermentation inhibitors. These include acetic acid, furan compounds (furfural and 5-hydroxy methyl furfural) and several products of lignin oxidation and degradation [3]. Some of these are in the colloidal phase whereas others, particularly the small molecule organics are in the solution phase. The colloidal particles not only inhibit the fermentation activities of microorganisms but also foul any filtration membranes used for separation and purification of extracts [4, 5]. Therefore, processes to separate such compounds from extracts are necessary and critical for viable biorefinery processes.
A hot water or acid hydrolysis may be followed by an enzyme hydrolysis to break down complex carbohydrates into fermentable monosaccharides and disaccharides. Commercially available hydrolysis enzymes include Cellic® HTec3, a concentrated hemicellulase that works alone or in combination with Cellic® CTec3 cellulase enzyme from Novozymes (Denmark). See: Zhang, Yi-Heng Percival, and Lee R. Lynd, “Toward an aggregated understanding of enzymatic hydrolysis of cellulose: noncomplexed cellulase systems.” Biotechnology and bioengineering 88.7 (2004): 797-824; Fan, L. T., Yong-Hyun Lee, and David H. Beardmore. “Mechanism of the enzymatic hydrolysis of cellulose: effects of major structural features of cellulose on enzymatic hydrolysis.” Biotechnology and Bioengineering 22.1 (1980): 177-199, Mandels, Mary, Lloyd Hontz, and John Nystrom. “Enzymatic hydrolysis of waste cellulose.” Biotechnology and Bioengineering 16.11 (2004): 1471-1493; Philippidis, George P., Tammy K. Smith, and Charles E. Wyman. “Study of the enzymatic hydrolysis of cellulose for production of fuel ethanol by the simultaneous saccharification and fermentation process.” Biotechnology and bioengineering 41.9 (1993): 846-853; Pääkkö, M., et al. “Enzymatic hydrolysis combined with mechanical shearing and high-pressure homogenization for nanoscale cellulose fibrils and strong gels.” Biomacromolecules 8.6 (2007): 1934-1941; Yang, Bin, and Charles L. Wyman. “BSA treatment to enhance enzymatic hydrolysis of cellulose in lignin containing substrates.” Biotechnology and Bioengineering 94.4 (2006): 611-617; Sun, Ye, and Jiayang Cheng. “Hydrolysis of lignocellulosic materials for ethanol production: a review.” Bioresource technology 83.1 (2002): 1-11; Saddler, J. N., et al. “Enzymatic hydrolysis of cellulose and various pretreated wood fractions.” Biotechnology and bioengineering 24.6 (1982): 13894402. Khodaverdi, Mandi, et al. “Kinetic modeling of rapid enzymatic hydrolysis of crystalline cellulose after pretreatment by NMMO.” Journal of industrial microbiology & biotechnology (2012): 1-10; Mama, Patrick, et al. “Combination of enzymatic hydrolysis and ethanol organosolv pretreatments: Effect on lignin structures, delignification yields and cellulose-to-glucose conversion.” Bioresource Technology (2012); Wiman, Magnus, et al. “Cellulose accessibility determines the rate of enzymatic hydrolysis of steam-pretreated spruce.” Bioresource Technology (2012); Elliston, Adam, et al. “High concentrations of cellulosic ethanol achieved by fed batch semi simultaneous saccharification and fermentation of waste-paper.” Bioresource Technology (2013); Kinnarinen, Teemu, et al. “Effect of mixing on enzymatic hydrolysis of cardboard waste: Saccharification yield and subsequent separation of the solid residue using a pressure filter.” Bioresource technology (2012); Wang, Lei, Richard Templer, and Richard J. Murphy. “High-solids loading enzymatic hydrolysis of waste papers for biofuel production.” Applied Energy (2012); Li, Sujing, Xiaonan Zhang, and John M. Andresen. “Production of fermentable sugars from enzymatic hydrolysis of pretreated municipal solid waste after autoclave process.” Fuel 92.1 (2012): 84-88; Dubey, Alok Kumar, et al. “Bioethanol production from waste paper acid pretreated hydrolyzate with xylose fermenting Pichia stipitis.” Carbohydrate Polymers (2012); Kinnarinen, Teernu, et al. “Solid-liquid separation of hydrolysates obtained from enzymatic hydrolysis of cardboard waste.” Industrial Crops and Products 38 (2012): 72-80; Nørholm, Nanna Dreyer, Jan Larsen, and Frank Krogh Iversen. “Non-pressurised pre-treatment, enzymatic hydrolysis and fermentation of waste fractions.” U.S. patent application Ser. No. 13/405,262; Das, Arpan, et al. “Production of Cellulolytic Enzymes by Aspergillus fumigatus ABK9 in Wheat Bran-Rice Straw Mixed Substrate and Use of Cocktail Enzymes for Deinking of Waste Office Paper Pulp.” Bioresource technology (2012). Chen, Hui, et al. “Enzymatic Hydrolysis of Recovered Office Printing Paper with Low Enzyme Dosages to Produce Fermentable Sugars,” Applied biochemistry and biotechnology (2012): 1-16. Yan, Shoubao, et al. “Fed batch enzymatic saccharification of food waste improves the sugar concentration in the hydrolysates and eventually the ethanol fermentation by Saccharomyces cerevisiae H058.” Brazilian Archives of Biology and Technology 55.2 (2012): 183-192; Arora, Anju, et al. “Effect of Formic Acid and Furfural on the Enzymatic Hydrolysis of Cellulose Powder and Dilute Acid-Pretreated Poplar Hydrolysates.” ACS Sustainable Chemistry & Engineering 1.1 (2012): 23-28; Wang, Lei, et al. “Technology performance and economic feasibility of bioethanol production from various waste papers.” Energy & Environmental Science 5.2 (2012): 5717-5730; Vazana, Yael, et al. “Designer Cellulosomes for Enhanced Hydrolysis of Cellulosic Substrates.” Cellulases (2012): 429; Van Dyk, J. S., and B. I. Pletschke, “A review of lignocellulose bioconversion using enzymatic hydrolysis and synergistic cooperation between enzymes—Factors affecting enzymes, conversion and synergy.” Biotechnology Advances (2012); Menind, A., et al. “Pretreatment and usage of pulp and paper industry residues for fuels production and their energetic potential.” International Scientific Conference Biosystems Engineering, Tartu, Estonia, 10-11 May 2012. Vol. 10. No. Special Issue I. Estonian Research Institute of Agriculture, 2012; Han, Lirong, et al. “Alkali pretreated of wheat straw and its enzymatic hydrolysis.” Brazilian Journal of Microbiology 43.1 (2012): 53-61; Holm, Jana, et al. “Pretreatment of fibre sludge in ionic liquids followed by enzyme and acid catalysed hydrolysis.” Catalysis Today (2012), each of which is expressly incorporated herein by reference.
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One of the major obstacles to the large scale industrial fermentation of hydrolyzates is the lack of efficient and cost effective separation and purification methods. The major constituents of woody biomass (cellulose, hemicellulose and lignin) cannot be isolated simultaneously as polymers and several processes must be employed involving the degradation of at least one polymer. One approach to solving the obstacle problems is to initially treat the biomass to degrade hemicellulose by extraction or autohydrolysis (hot water extraction process) in the absence of mineral acids or caustics thereby leaving both cellulose and lignin as essentially undegraded polymers [50, 51]. Autohydrolysis is of interest because water and biomass are the only reagents, without complicating and costly side reaction.
A review of possible separation techniques useful in biorefineries was recently published by Huang et al [6]. Detoxification methods include extraction [7], overliming [8, 9], adsorption on zeolites [10, 11], activated carbon [12], the application of ion exchange resins [13] and hybrid processes such as adsorptive membranes [14]. Filtration is one such alternative, followed by liquid-liquid extraction for separating acetic acid and furfural. Reverse osmosis membranes have also been applied to separate acetic acid and furfural from the extracts to yield a concentrated hemicellulose rich retentate and a dilute acetic acid permeate. Fouling of the membranes used in nanofiltration or reverse osmosis is a serious problem leading to decaying permeate fluxes and renders the separation uneconomical on large scale [5].
Polyelectrolytes have been used in the past for the clarification of lignocellulosic suspensions to enhance solid liquid separations. Hydrolyzates produced by hot-water treatment of sugar maple (Acer saccharum) wood chips were flocculated by the application of a cationic polymer—poly-diallyl dimethyl ammonium chloride (pDADMAC) [15]. The hydrolyzates were highly turbid (>10000 NTUs) and the average particle size ranged from ˜220 nm to 270 nm, the larger particles obtained from more severe treatments. The effect of polymers on the colloidal stability depends on the specifics of adsorption of the polymer on the colloidal particles [19, 20].
Polymers flocculate colloidal suspensions generally through the mechanisms of charge neutralization, formation of patches of opposite charge and subsequent attraction (referred to as patching) and bridging [see e.g. 43]. Flocculation depends on the size of the polymer molecule both in solution and after adsorption (its conformation), charge density, polymer concentration, presence of other electrolytes and the mode of addition [21-29]. Poly-ethylene imine (PEI), and (pDADMAC) are low molecular weight and high charge density polymers which act by forming cationic patches on particles resulting in attractive interactions between colloidal particles [30, 31]. The introduction of cationic countercharges reduces the extent of the electrical double layers and also contributes to the flocculation process. Cationic polyelectrolytes are subject to changes in charge and size in solution upon alteration of pH and ionic strength. Furthermore, the adsorption of the polyelectrolytes on an oppositely charged surface may change with these solution properties. Since these polymers are polybase, addition of protons (reduction in pH) will result in protonation and subsequent expansion due to repulsion [32, 33]. High molecular weight cationic Poly acrylamide (CPAM) flocculates suspensions by adhering to particle surfaces and forming a bridge between them [34-37]. It is also known that the mechanism of flocculation, i.e. patching or bridging effects the rate and extent of dewatering achieved, i.e. the dynamics of the fluid-particle phase separation processes such as filtration and sedimentation.
The raw extracts from woody biomass (which woody biomass itself is separately used as a feedstock for pulp paper production) consist of water soluble and insoluble substances, mostly as monomers and oligomers of sugars, acetic acid, methanol, aromatic compounds, other low molecular weight extractable substances and fractions of lignin and residual particles [49,51].
These raw extracts, produced by water treatment, contain significant quantities of colloidal material or particulates composed mostly of lignin and its derivatives. The colloidal particles not only inhibit the fermentation activities of some microorganisms, but also foul any filtration membranes used for separation and purification of extracts. Separation and purification of these contaminant components from hot water extracts is an important step in separation processes of the biorefinery industry.
The particulate phase of the wood extracts, containing colloidal particles which foul and plug membranes used in the separation and purification of wood extracts [52, 53], comprise suspended colloidal particles constantly and randomly bombarded from all sides by molecules of the liquid, making them move in a zigzag path. This type of movement is known as Brownian motion and increases in significance, as particle size decreases. Since the mass of a colloidal particle is also small, its settling rate under the influence of gravity is slow. When the effect of Brownian motion dominates, it becomes very difficult and an unacceptably slow process to separate the particles from the liquid by gravity sedimentation [52, 53].
Colloidal particles, usually anionically charged, which cannot be removed from a liquid by sedimentation within a short period of time (less than few hours), are typically converted into aggregates by coagulation or flocculation. The larger aggregates have more mass and the influence of gravity dominates over Brownian motion so that sedimentation occurs in a relatively short time. Flocculation and sedimentation of colloidal suspensions play an important role in separation of solids particles from liquid media. The particulate phase in wood extracts can be separated by treating with polyelectrolyte flocculating agents. Polymer induced flocculation is also used to enhance the separation of colloidal particles in wood extracts [48, 49].
The effect of polymers on colloidal stability depends on the peculiarities of the colloidal particle adsorption to the surface of the polymer. The polymers can destabilize the colloidal particles through charge neutralization, electrostatic patch and bridging flocculation. The time dependence and efficiency of the flocculation process is a function of many variables, including the structure of the molecule, its molecular mass, charge density, concentration of the polymer solution, content of the electrolytes, and the mode of addition of polymer solution to suspension [54].
Autohydrolysis (hot water extraction) is of interest because water and biomass are the only reagents. The raw extracts from woody biomass consists of water soluble and insoluble substances, mostly as monomers and oligomers of sugars, acetic acid, methanol, aromatic compounds, other low molecular weight extractable substances and fractions of lignin and residual particles. Raw extracts produced by water treatment contains significant quantities of colloidal material. These particulates are composed mostly of lignin and its derivatives. Flocculation and sedimentation of colloidal suspensions play an important role in separation of solids particles from liquid media.
Hydrolyzates produced by pretreatment of lignocellulosic materials contain significant colloidal material that is anionically charged. Many of the compounds that are present in the hydrolyzates are inhibitory to fermentation and interfere with downstream separations. The flocculation of this colloidal material makes separations easier by sedimentation and can reduce the fouling tendencies of membranes. It can also reduce the toxicity of the hydrolyzates to fermentation microorganisms.
The interaction of PEO with modified lignin-type compositions has been studied in the past [53, 54, 55, 56, 57]. PEO is able to adsorb on unbleached Kraft or sulphite fibers (i.e., wood biomass modified by the Kraft or sulfite process), latex or clay without any cofactor, but does not adsorb on other particles such as calcium carbonate and bleached Kraft fibers by itself. In the latter cases, it is necessary to use another compound that interacts with PEO and the mineral surfaces. Such compounds are called cofactors and normally have aromatic groups in them [53, 54, 57]. The PEO polymer is able to form hydrogen bonds with other electron acceptor compounds because of the unshared electron pair of the ether oxygen. The formation of complexes between PEO and lignin has been described as a complex bridging association-induced flocculation [53, 54, 55, 56, 57]. PEO was therefore used in the paper making process and the treatment of charge-modified solid biomass.
FIG. 16 shows example process steps in a biomass processing system.
FIG. 19 shows a composition analysis of hot water extracted woodchips.
TABLE 1Comparison of various technologies:SeparationTempProcess;HydrolyzateFlocculantChargeInvestigatorDosage(° C.)pHEfficiencyRemarksWoodpDADMACCationicDuarte,0-47.3253.5HighlyFlocculation andHydrolyzate (10xB. V. RamaraoppmEfficientSedimentation;Diluted)(1)ExpensiveWoodCPAMCationicR Singh,0-200253.5none- lowFlocculation andHydrolyzateB. V. RamaraoppmefficiencyUV Analysis;(2)Low CostSyntheticPEICationicCarter,0.1-1223.4Flocculation/solution (xylose,MenkhausMolar Eq.adsorption andglucose, HMF(3)filtration andFurfural,centrifugationdiethylamine)LignocellulosicKemiraCationicBurke,1000-225  largeFlocculation andslurries (PineC1592Menkhaus5000Filtration,Wood(4)mg/Lcentrifugationhydrolyzates)Superfloc C-C1594Cationicmedium1592 PGPolyacrylamideC1598Cationicmedium[PC]C1594 Kemira130AnioniclowGVHRS140AnioniclowGVHRSAnionic PAMA 1883 RSAnioniclowAnionic PAMA1849 RSAnioniclowNonionic PAMN 1986NeutrallowCorn grainKemiraCationicMenkhaus et0-5.6224  Flocculation andStillage LiquidC1592al., (5)mg/gCentrifugation/StreamFiltrationCationic PAMC 4512CationicCationic PAMC 4516CationicAnionic PAMA1883AnionicAnionic PAMA 130AnionicAnionic PAMA 140AnionicNonionic PAMN 1986NeutralPre HydrolysisPEOShi, Ni (6)0-350N/A3.7-CombinedLiquor (Frommg/L1.5Acidification/PEOKraft BasedFlocculation anddissolving pulpcentrifugationProductionProcess)Pre HydrolysisPEOShi, Ni (7)10-100RT2  noAcidification andLiquor (Liquorppmseparation -flocculationproduced afternegligiblekraft pulpingfrom bottom ofdigestor)PAC (Poly Aluminum100 ppmRT2  noChloride)separationEC (Ethyl Acetate)1.5%-4%RT2  mediumWoodpDADMACCationicL R Yasarla,0-15015-253.5-HighlyFlocculation andHydrolyzateB. V. Ramaraoppm8.0EfficientSedimentationExpensiveWoodAlumPoly-L R Yasarla,0.01-15-253.5-Medium -Flocculation andHydrolyzateelectrolyteB. V. Ramarao0.25M8.0highSedimentation;low costWoodPEICationicL R Yasarla,0-15015-253.5-HighlyFlocculation andHydrolyzateB. V. Ramaraoppm8.0EfficientSedimentation;low costWoodPEONeutralL R Yasarla,0-40 ppm15-253.5-NoFlocculation andHydrolyzateMediumB. V. Ramarao8.0SeparationSedimentation;MWlow costWoodPEONeutralL R Yasarla,0-15015-253.5HighlyFlocculation andHydrolyzateHigher MWB. V. RamaraoppmEfficientSedimentationWoodCPAMCationicL R Yasarla,0-25 ppm15-252-HighlyFlocculation andHydrolyzateMediumB. V. Ramarao8.5EfficientSedimentation;MWlow costWoodCPAMCationicL R Yasarla,0-30 ppm15-253.5HighlyFlocculation andHydrolyzateHigher MWB. V. RamaraoEfficientSedimentationWoodC-StarchCationicL R Yasarla,0-75 g/15-253.5NoFlocculation andHydrolyzateB. V. Ramarao100 mlSeparationSedimentationWoodAlum +L R Yasarla,0.15M +15-253.5EfficientFlocculation andHydrolyzatePEIB. V. Ramarao25 ppmSedimentation;low costWoodAPAMAnionicL R Yasarla,0-40 ppm15-253.5UnderFlocculation andHydrolyzateB. V. RamaraoInvestigationSedimentation(1) Bioresource Technology (submitted 29 Sep. 2009, accepted 26 May 2010).(2) SUNY-ESF New Technology Disclosure 2008(3) Biotechnology and Bioengineering (submitted 29 Nov. 2010, accepted 14 Mar. 2011)(4) Biomass and Bioenergy (submitted 13 Aug. 2009, accepted 20 Aug. 2010)(5) Bioresource Technology (submitted 23 Feb. 2009, accepted 2 Nov. 2009)(6) Bioresource Technology (submitted 18 Dec. 2010, accepted 24 Jan. 2011)(7) Bioresource Technology (submitted 24 Apr. 2010, accepted 18 Aug. 2010)